Image reading apparatus

ABSTRACT

An image reading apparatus includes a drive source, a speed change mechanism provided in a transmission unit that transmits a driving force from the drive source to a scanning drive unit. The speed change mechanism is configured to change a transmission ratio of the transmission unit by selectively bringing one of first and second couplings into a connected state. The image reading apparatus further includes a selector configured to selectively bring one of the first and second couplings into the connected state, a detector configured to detect the connected state of the second coupling, and a controller configured to control a scanning position of the reading unit based on the transmission ratio changed via the second coupling after the detector detects the connected state of the second coupling after the selector changes selection from the first coupling to the second coupling.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image reading apparatus configuredto read an image of a document placed on a document positioning plateand to a reading method therefor.

2. Description of the Related Art

FIG. 9 illustrates a configuration of a conventional color image readingapparatus.

A conventional image reading apparatus, such as the one discussed inJapanese Patent Application Laid-Open No. 2000-13564, reads an image ofa document 96 by causing a reading unit 91 to scan the document 96 inparallel with a document positioning glass plate 94. The reading unit 91includes a linear image sensor. A timing belt 93, which transmits powerfrom a stepping motor 92 serving as a scanning drive source, is fixed tothe reading unit 91.

Forward/reverse rotation of the stepping motor 92 enables the readingunit 91 to reciprocatingly scan an area of the document positioningglass plate 94. When reading an image of the document 96, the imagereading apparatus drives the stepping motor 92 at a driving speedcorresponding to a given resolution to cause the reading unit 91 toperform movement scan.

Generally, the minimum value of an exposure time needed to obtaininformation representing one line of the document 96 in reading an imageof the document 96 is determined according to both the sensitivity ofthe image sensor and an amount of light received by the image sensor.The maximum value of a movement speed of the reading unit 91 can bedetermined according to the minimum value of the exposure time needed toobtain such information. When reading an image at low resolution, thereading unit 91 can set a movement speed at a high value. When readingan image at high resolution, it is necessary that the movement speed ofthe reading unit 91 is low. Actually, the upper limit of the movementspeed is subject to various constrains, such as a time in which anelectric signal is processed.

A stepping motor, which is useful for easy position control, is used todrive movement scan of the reading unit 91 of the image readingapparatus, and is required to rotate at a speed corresponding to areading resolution. In a case where an image reading apparatus is beingused in an electric power supply limited environment, e.g., where theimage reading apparatus is USB (universal serial bus)-powered, the imagereading apparatus may be unable to provide a desired reading speed rangeusing a motor having a narrow speed range. Particularly, it is difficultwithin a range of a rotational speed of a single motor in a conventionalhigh-resolution image reading apparatus to scan an image at readingspeeds respectively corresponding to various resolutions. Accordingly, aspeed change mechanism, such as a gear change, can be used. In a casewhere a driving force provided by a single drive source is transmittedusing the speed change mechanism, such as a gear change, a state ofgears varies during the gear change. Thus, the conventional imagereading apparatus cannot accurately achieve position control accordingto the number of steps of a stepping motor.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, an image readingapparatus configured to read an image of a document by causing ascanning drive unit to perform scanning drive of a reading unit includesa drive source, a speed change mechanism provided in a transmission unitthat transmits a driving force from the drive source to the scanningdrive unit and configured to change a transmission ratio of thetransmission unit by selectively bringing one of first and secondcouplings into a connected state, a selector configured to selectivelybring one of the first and second couplings into the connected state, adetector configured to detect the connected state of the secondcoupling, and a controller configured to control a scanning position ofthe reading unit by applying a drive control signal to the drive source.The controller starts controlling the scanning position of the readingunit based on the transmission ratio changed via the second couplingafter the detector detects the connected state of the second couplingafter the selector changes selection from the first coupling to thesecond coupling.

Further features and aspects of the present invention will becomeapparent from the following detailed description of exemplaryembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate exemplary embodiments, features,and aspects of the invention and, together with the description, serveto explain the principles of the invention.

FIG. 1 is a perspective view illustrating an image reading apparatusaccording to an exemplary embodiment of the present invention in a statein which a document cover is opened.

FIG. 2 is a block diagram of an image reading apparatus according to anexemplary embodiment of the present invention.

FIG. 3 illustrates a state in which a gear drive train having a smallspeed reduction ratio is selected.

FIG. 4 illustrates a state in which a gear drive train having a largespeed reduction ratio is selected.

FIG. 5 illustrates a configuration of a gear drive train according to anexemplary embodiment of the present invention.

FIG. 6 is a cross-sectional view of a speed change mechanism accordingto an exemplary embodiment of the present invention.

FIG. 7 illustrates rotational speed ranges respectively provided by geardrive trains corresponding to different speed reduction ratios.

FIG. 8 is a flowchart illustrating an operation of an image readingapparatus according to an exemplary embodiment of the present invention.

FIG. 9 illustrates a conventional image reading apparatus.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Various exemplary embodiments, features, and aspects of the inventionwill be described in detail below with reference to the drawings.

First Exemplary Embodiment

FIG. 1 illustrates a configuration of a color image reading apparatusaccording to a first exemplary embodiment of the present invention. Asillustrated in FIG. 1, a document is set on a document positioning glassplate 2 mounted on the top surface of an apparatus body 1. A contactimage sensor 3 scans the document in parallel with the documentpositioning glass plate 2 and reads an image of the document. Thecontact image sensor 3 includes light-emitting diodes (LEDs) serving aslight sources which illuminate a document and respectively correspond tothree colors, a rod lens array which forms an image on a light receivingelement of an image sensor from light reflected by the document, and theimage sensor. The contact image sensor 3 constitutes a reading unit.

The contact image sensor 3 is supported on a carriage 5, which slides ona guide shaft 4 fixed to the apparatus body 1. A timing belt 15 is fixedto the carriage 5. The timing belt 15 serves as a scanning drive unitwhich transmits power from a stepping motor 17 serving as a drive sourceto the reading unit via a gear drive train 16 serving as a transmissionunit. The gear drive train 16 includes a speed change mechanism with agear change. A flexible cable (not shown) for inputting and outputtingelectric signals to and from the contact image sensor 3 is connected tothe contact image sensor 3 at one end thereof and to a control board(not shown) of the apparatus body 1 at the other end thereof.

In addition to the above-described components, the image readingapparatus includes an electric equipment unit including a control boardand a power supply, as constituent elements. These constituent elementsare disposed in the apparatus body 1, which fixedly supports thedocument positioning glass plate 2.

A document cover 6 serving as a cover member for pressing a documentonto the document positioning glass plate 2 is openably and closablyattached to the apparatus body 1 via a hinge 7. A document pressingsheet 8 is pasted to an inner surface of the document cover 6, whichfaces the document positioning glass plate 2.

FIG. 2 illustrates an electrical configuration of the image readingapparatus according to an exemplary embodiment. The contact image sensor3 includes a light source 31, serving as a reflective-documentilluminating light source, and an image sensor 32. A home positionsensor 18 connected to a system controller 20 is used to adjust aninitial position of the stepping motor 17, which performs scanning driveof the carriage 5. The system controller 20 controls the light source31.

The system controller 20 also controls the image sensor 32. An analogsignal processing circuit 21 processes an output signal from the imagesensor 32. An A/D conversion circuit 22 converts the processed outputsignal into a digital signal. An image processing circuit 23 processesthe digital signal. A buffer memory 24 stores the processed digitalsignal. The system controller 20, the buffer memory 24, and an interface25 are connected to one another and can perform data communication thereamong. The signal stored in the buffer memory 24 can be sent, as dataimage, to an external equipment 26 via the interface 25. The systemcontroller 20 is also connected to and controls the stepping motor 17, agear change motor 114, and a gear change sensor 116. The gear changesensor 116 detects a connected state of the gear drive train. The systemcontroller 20 controls the position of the carriage 5 with the steppingmotor 17.

The gear drive train 16 (shown in FIG. 1), serving as a transmissionunit, is described with reference to FIGS. 3 to 6. The gear drive train16 changes over between two couplings serving as a speed changemechanism.

A speed reduction ratio is defined as the ratio of the number ofrevolutions of a pulley 107 c driving the timing belt 15 to the numberof revolutions of the stepping motor 17. The speed reduction ratio forlow-resolution reading with high-speed movement of the reading unit is “1/15”. The speed reduction ratio for high-resolution reading withlow-speed movement of the reading unit is “ 1/60”. These ratios aretransmission ratios at which a driving force is transmitted from thestepping motor 17 to the reading unit to perform movement scan. That is,an advancing amount of the carriage 5 generated by a low-resolutionpurpose gear drive train and corresponding to one pulse applied to thestepping motor 17 is equal to that of the carriage 5 generated by ahigh-resolution purpose gear drive train and corresponding to fourpulses applied to the stepping motor 17. Therefore, the high-resolutionpurpose gear drive train enables the reading unit to read an image in amovement direction of the carriage 5 four times finer than thelow-resolution purpose gear drive train.

An exemplary embodiment assumes that the reading unit reads an imagewith 4800 scanning lines per inch at a maximum. Accordingly, a movementdistance of the carriage 5 generated by the high-resolution purpose geardrive train and corresponding to one pulse applied to the stepping motor17 is equal to the length of one pixel in the case of reading an imageat a resolution of 4800 dpi. In the case of using this gear drive train,two pulses applied to the stepping motor 17 correspond to a resolutionof 2400 dpi. Three pulses applied to the stepping motor 17 correspond toa resolution of 1600 dpi.

A gear ratio of the low-resolution purpose gear drive traincorresponding to a small speed reduction ratio is ¼ times that of thehigh-resolution purpose gear drive train corresponding to a large speedreduction ratio. A movement distance of the carriage 5 generated by thelow-resolution purpose gear drive train and corresponding to one pulseapplied to the stepping motor 17 is equal to the length of one pixel inthe case of reading an image at a resolution of 1200 dpi.

FIGS. 3 to 5 illustrate the gear drive train according to an embodiment.FIG. 6 illustrates a cross-section taken along line A-A′ shown in FIG.3.

The small-speed-reduction-ratio gear drive train includes a drive gear17 a attached to the stepping motor 17, a gear 101, an idler gear 102, agear 103, a gear 104, a gear 105, and a gear 107 a. Thelarge-speed-reduction-ratio gear drive train includes gears 106 and 107b in addition to the drive gear 17 a and the gears 101 to 104.

These gear drive trains differ from each other in the gear ratio of thefinal stage. The gear ratio between the gears 105 and 107 a is “2”. Thegear ratio between the gears 106 and 107 b is “½”.

The gears 104, 105, and 106 are coaxial with a shaft 130. The gear 104always meshes with the gear 103 and can move axially (i.e., upward anddownward, as viewed in FIG. 6). A small-diameter gear of the gear 103has a thickness corresponding to an axial movement distance of the gear104. The gears 105 and 106 are fixed in axial position. The gears 105and 106 always mesh with the gears 107 a and 107 b, respectively.

As illustrated in FIG. 6, coupling portions 104 a and 106 a of acoupling are provided on the top surface of the gear 104 and the bottomsurface of the gear 106, respectively, to connect the gears 104 and 106to each other. Similarly, coupling portions 104 b and 105 a of anothercoupling are provided on the bottom surface of the gear 104 and the topsurface of the gear 105, respectively, to connect the gears 104 and 105to each other. A shaft of the gear 104 serves as an output shaft thatselectively transmits a driving force from the stepping motor 17 to oneof the two couplings.

Each of the couplings according to an exemplary embodiment includes aninternal gear and a spur gear, which are equal to each other in thenumber of teeth. The gears 104 a and 104 b are internal gears, and thegears 105 a and 106 a are spur gears, whose tooth shapes are not shown.The spur gear can be inserted into the internal gear, so that thesegears mesh with each other to connect associated components to eachother.

A change arm 110 is located between the gears 104 and 105 coaxially withthe shaft 130. A spring 111 presses the gear 104 against the change arm110. A spring 112 pushes the change arm upward as viewed in FIG. 6. Aspring 113 always presses the change arm 110 against a gear change wheel115 along a shaft 131.

In a free state, a resultant of forces of the springs 112 and 113 isgreater than a force of the spring 111. Thus, the gears 104 and 106 areconnected to each other at the coupling portions 104 a and 106 a.

Cam surfaces 110 a and 115 a are respectively provided on a surface ofthe change arm 110 and a surface of the gear change wheel 115, againstwhich the change arm 110 is pressed. When a gear change motor 114 isdriven, a gear change worm gear 114 a serving as a drive gear attachedto the gear change motor 114 rotates the gear change wheel 115. Then,the gear change wheel 115 lowers the cam surface 110 a, with which thespiral cam surface 115 a engages, to change the height of the change arm110. A selector having such a configuration can select a rotationdirection of the gear change motor 114 to select one of the couplings sothat the coupling portions of the selected coupling are connected toeach other. The spring 111 presses the gear 104 downward as viewed inFIG. 6 against the change arm 110. Thus, as the change arm 110 moves,the gear 104 moves downward as viewed in FIG. 6. In a case illustratedin FIG. 3, the coupling portion 104 b of the gear 104 is connected tothe coupling portion 105 a of the gear 105 in a state in which thechange arm 110 and the gear 104 are placed at an axially lower side ofthe shaft 130, as viewed in FIG. 6. Thus, a small-speed-reduction-ratiogear drive train is selected. In a case illustrated in FIG. 4, thecoupling portion 104 a of the gear 104 is connected to the couplingportion 106 a of the gear 106 in a state in which the change arm 110 andthe gear 104 are placed at an axially upper side of the shaft 130, asviewed in FIG. 6. Thus, a large-speed-reduction-ratio gear drive trainis selected.

The gear change sensor 116 can detect the axial position of the changearm 110. The gear change sensor 116 serves as a detector that detects agear change. When the coupling portion 104 a of the gear 104 and thecoupling portion 106 a of the gear 106 are connected to each other, alight-blocking plate 116 a mounted on the change arm 110 blocks lightfrom entering a photointerrupter 116 b mounted on a fixed portion.

The gears 107 a, 107 b, and the pulley 107 c are connected together androtate forward and backward as one body. The timing belt 15 is attachedto the pulley 107 c. Thus, a scanning drive unit is constituted, whichtransmits the rotation of the pulley 107 c to the carriage 5.Consequently, the scanning drive unit transmits to the reading unit adriving force output from the stepping motor 17 to perform the scanningdrive of the reading unit.

A transition from a state in which the transmission unit operates withthe small-speed-reduction-ratio gear drive train to another state inwhich the transmission unit operates with thelarge-speed-reduction-ratio gear drive train is described below. In thestate in which the transmission unit operates with thesmall-speed-reduction-ratio gear drive train, the coupling portions 104b and 105 a are connected to each other. Power from the gear 104 istransmitted to the gear 105. At that time, the photointerrupter 116 b ofthe gear change sensor 116 is not light-blocked

1) First, the home position of the carriage 5 is detected. A counterprovided in the system controller 20 to control the stepping motor 17 isinitialized. Consequently, the scanning position of the reading unit canbe initialized. The home position is located upstream in areading/scanning direction in which reading/scanning of a document isperformed. The home position is detected while the carriage 5 is movedin the reading/scanning direction during the forward rotation of thestepping motor 17.2) Next, the carriage 5 is moved to an immediate front of a readingstart position with the small-speed-reduction-ratio gear drive train.Then, the stepping motor 17 is stopped.3) In order to select the large-speed-reduction-ratio gear drive train,the gear change motor 114 is caused to perform forward rotation. Thus,the gear change wheel 115 rotates in the direction of arrow “b” shown inFIG. 4. Consequently, engaging surface parts of the spiral cam surface115 a and the cam surface 110 a, which engage with each other, aregradually slid away from each other. Accordingly, the change arm 110moves upward as viewed in FIG. 6. Also, the gear 104 moves axiallyupward as viewed in FIG. 6. As the gear 104 moves, the coupling portions104 b and 105 a are disconnected.4) When the gear change wheel 115 is further rotated in the direction ofarrow “b”, the top surface of the coupling portion 104 a collides withthe bottom surface of the coupling portion 106 a. In a case where thegear teeth are in phase, the coupling portions 104 a and 106 a are wellconnected to each other. However, usually, the gear teeth are out ofphase, so that the top surface of the coupling portion 104 a collideswith the bottom surface of the coupling portion 106 a.5) In a case where the gear teeth are in phase, the gear change wheel115 is further rotated in the same direction to an angular position sothat the coupling portions 104 a and 106 a are well connected to eachother. However, usually, the gear teeth are out of phase, the engagingsurface parts of the spiral cam surface 115 a and the cam surface 110 aare disengaged from each other. Thus, the cam surface 115 a disengagesupward from the cam surface 110 a. Conversely, in a case where the gearteeth are in phase, the photointerrupter 116 b of the gear change sensor116 is light-blocked. Thus, a gear change to thelarge-speed-reduction-ratio gear drive train is detected. Consequently,the gear change is finished.6) In a case where the gear change sensor 116 can detect no gear change,the top surface of the coupling portion 104 a collides with the bottomsurface of the coupling portion 106 a. The gear 104 is upward pushed viathe change arm 110 by an upward resultant of forces of the springs 112and 113, which is larger than the downward force of the spring 111.Then, the forward rotation of the stepping motor 17 is resumed.7) The coupling portion 104 a rotates in a state in which the topsurface of the coupling portion 104 a is in contact with the bottomsurface of the coupling portion 106 a. When the gear teeth become inphase, the gear 104 moves upward. The gear 104 is thus connected by thecoupling portions 104 a and 106 a to the gear 106, which serves as afirst transmission gear. At that time, the gear change sensor 116detects a gear change. Thus, the gear change is finished.

Subsequently, the rotation of the stepping motor 17 is transmitted tothe pulley 107 c via the gears 106 and 107 b. At that time, the gear 107a rotating together with the gear 107 b and the pulley 107 c as one bodycauses the gear 105 to run idle.

Conversely, in order to select the small-speed-reduction-ratio geardrive train, first, the rotation of the stepping motor 17 is stopped.Next, the gear change motor 114 is rotated backward. Thus, the gearchange wheel 115 rotates in the direction of arrow “a” shown in FIG. 4.Consequently, when the change arm 110 moves downward as viewed in FIG.6, a force of the spring 111 causes the gear 104 to move axiallydownward as viewed in FIG. 6. Accordingly, the coupling portions 104 aand 106 a are disengaged from each other. At that time, thephotointerrupter 116 b of the gear change sensor 116 is unshielded fromlight. However, the coupling portions 104 b and 105 a are not yetconnected to each other. When the gear change wheel 115 further rotatesin the direction of arrow “a” shown in FIG. 4, the change arm 110 movesdownward. Usually, the gear teeth are out of phase, so that the bottomsurface of the coupling portion 104 a collides with the top surface ofthe coupling portion 105 a. When the gear change wheel 115 furtherrotates in the direction of arrow “a” shown in FIG. 4, the change arm110 moves downward to a position at which the coupling portions 104 band 105 a can fully be connected to each other in a case where the gearteeth are in phase. Subsequently, when the stepping motor 17 is causedto run, the coupling portion 104 a rotates in a state in which thebottom surface of the coupling portion 104 b is in contact with the topsurface of the coupling portion 105 a. When the gear teeth become inphase, a force of the spring 111 causes the gear 104 to move downward,so that the coupling portions 104 b and 105 a are connected to eachother. Subsequently, the rotation of the stepping motor 17 istransmitted to the pulley 107 c via the gear 105 and the gear 107 a. Atthat time, the gear 107 b rotating together with the gear 107 a and thepulley 107 c as one body causes the gear 106 to run idle.

As described above, the axial position of the gear 104 can be changed bychanging the axial position of the change arm 110. The two gear drivetrains can selectively be used by changing the axial position of thegear 104.

Next, an operation of the image reading apparatus for reading a documentat a high resolution and an operation of the image reading apparatus forreading a document at a low resolution according to an exemplaryembodiment are described below.

First, an operation of the image reading apparatus for reading adocument at a low resolution is described.

The carriage 5 is placed at an initial position. Then, the gear changesensor 116 confirms that the gear 104 is connected to the gear 105.Subsequently, the stepping motor 17 is driven to move the carriage 5from the initial position to the reading start position. The carriage 5can be moved at high speed to the reading start position. Accordingly,the low resolution purpose gear drive train corresponding to a smallspeed reduction ratio is used. Then, reading of a document is startedusing the low resolution purpose gear drive train corresponding to asmall speed reduction ratio. Upon completion of reading the document,the stepping motor 17 is rotated backward while the low resolutionpurpose gear drive train remains selected. Thus, the carriage 5 is movedto the initial position.

Next, an operation of the image reading apparatus for reading a documentat a high resolution is described.

First, the carriage 5 is placed at an initial position. Then, the gearchange sensor 116 confirms that the gear 104 is connected to the gear105. Subsequently, the stepping motor 17 is driven to move the carriage5 from the initial position to the reading start position. At that time,the low resolution purpose gear drive train corresponding to a smallspeed reduction ratio is used, similar to the case of reading a documentat a low resolution.

Subsequently, the stepping motor 17 is caused to rotate forward. Whenthe carriage 5 reaches the reading start position, the stepping motor 17is temporarily stopped.

Subsequently, the gear change motor 114 is rotated backward until thegear change sensor 116 confirms that the gear 104 is connected to thegear 106. The gear change wheel 115 is rotated in the direction of arrow“b” shown in FIG. 4. Consequently, the change arm 110 moves upward asviewed in FIG. 6. In synchronization with this movement, the gear 104 ismoved axially upward as viewed in FIG. 6. Then, the gear 104 isdisconnected from the gear 105 and is connected to the gear 106.Subsequently, the stepping motor 17 is driven to start reading thedocument at a high resolution. Upon completion of reading the document,the gear 104 is switched to the position corresponding to the lowresolution purpose gear drive train. Then, the stepping motor 17 isrotated backward to move the carriage 5 to the initial position.

FIG. 7 illustrates rotational speed ranges respectively provided by thegear drive trains corresponding to different speed reduction ratios. Asis understood from FIG. 7, according to an exemplary embodiment, a speedrange “A_” provided by the gear drive train corresponding to a speedreduction ratio of “ 1/15” and a speed range “C_” provided by the geardrive train corresponding to a speed reduction ratio of “ 1/60” can beselectively used. Accordingly, an available speed range “A+C” is acombination of the ranges “A_” and “C_”. In contrast, a conventionalapparatus employing a single gear drive train corresponding to a speedreduction ratio of, e.g., “ 1/30”, which is intermediate in valuebetween “ 1/15” and “ 1/60”, provides a very narrow speed range.

Second Exemplary Embodiment

Next, an image reading apparatus according to a second exemplaryembodiment is described below.

The image reading apparatus according to the second exemplary embodimentis configured such that the when a low resolution purpose gear drivetrain is changed to a high resolution purpose gear drive train, the lowresolution purpose gear drive train remains connected to the steppingmotor 17 until the high resolution purpose gear drive train is connectedthereto. Thus, when the stepping motor 17 is driven, the carriage 5 ismoved even before the high resolution purpose gear drive train isconnected to the stepping motor 17 after the change from the lowresolution purpose gear drive train to the high resolution purpose geardrive train is selected. The rest of the second exemplary embodiment issimilar to the first exemplary embodiment.

An operation of the image reading apparatus according to the secondexemplary embodiment is described below with reference to a flowchartillustrated in FIG. 8.

In step S1, the system controller 20 causes the gear change sensor 116to determine which of the high resolution purpose gear drive train andthe low resolution purpose gear drive train is connected to the steppingmotor 17. If the high resolution purpose gear drive train is connectedto the stepping motor 17, the gear change motor 114 is run to change thegear drive train to be connected to the stepping motor 17 from the highresolution purpose gear drive train to the low resolution purpose geardrive train.

In step S2, the system controller 20 causes the home position sensor 18to detect the initial position of the carriage 5. Then, the systemcontroller 20 adjusts the initial position to perform position control.

In step S3, the system controller 20 moves the carriage 5 to a gearchange position which is located at a front of a reading position. Inconsideration of a distance L by which the carriage 5 is moved during agear change, the carriage 5 is moved to the front of the readingposition rather than being moved just to the reading position.

In step S4, the system controller 20 runs the gear change motor 114 tochange the gear drive train to be connected to the stepping motor 17from the low resolution purpose gear drive train to the high resolutionpurpose gear drive train.

In step S5, the system controller 20 causes the stepping motor 17(carriage drive motor) to rotate by an angle corresponding to a minutestep.

In step S6, the system controller 20 causes the gear change sensor 116to determine whether the high resolution purpose gear drive train isconnected to the stepping motor 17. If the high resolution purpose geardrive train is connected to the stepping motor 17 (YES in step S6), thesystem controller 20 advances to step S7. Otherwise (NO in step S6), thesystem controller 20 returns to step S5.

In step S7, the system controller 20 can determine a distance, by whichthe carriage 5 has moved to perform the gear change, according to thenumber of times of performing step drive in step S5. The carriage 5 hasbeen stopped at the distance L in front of the reading position in stepS3. Thus, the remaining distance to the reading position can be obtainedby subtracting a distance, by which the carriage 5 has moved in step S5,from the distance L. Consequently, the system controller 20 controls thenumber of pulses serving as a drive control signal for driving thestepping motor 17, so that the carriage 5 moves the remaining distance.

In step S8, the system controller 20 reads an image.

In step S9, the system controller 20 runs the gear change motor 114 tochange the gear drive train to be connected to the stepping motor 17from the high resolution purpose gear drive train to the low resolutionpurpose gear drive train.

In step S10, the system controller 20 runs the stepping motor 17(carriage drive motor) to move the carriage 5 to the initial position.When the stepping motor 17 starts rotating, the gear drive train to beconnected to the stepping motor 17 is changed from the high resolutionpurpose gear drive train to the low resolution purpose gear drive train.

In step S11, the system controller 20 decelerates and stops the steppingmotor 17 when the home position sensor 18 detects the home position ofthe carriage 5.

In an exemplary embodiment of the present invention, the stepping motor17 can be replaced with a direct current (DC) motor with a rotaryencoder. In this case, the drive control of the DC motor is performedaccording to output pulses from the rotary encoder.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all modifications, equivalent structures, and functions.

This application claims priority from Japanese Patent Applications No.2006-205940 filed Jul. 28, 2006 and No. 2006-205947 filed Jul. 28, 2006,which are hereby incorporated by reference herein in their entirety.

1. An image reading apparatus configured to read an image of a documentby causing a scanning drive unit to perform scanning drive of a readingunit, the image reading apparatus comprising: a drive source; a speedchange mechanism provided in a transmission unit that transmits adriving force from the drive source to the scanning drive unit andconfigured to change a transmission ratio of the transmission unit byselectively bringing one of first and second couplings into a connectedstate; a selector configured to selectively bring one of the first andsecond couplings into the connected state; a detector configured todetect the connected state of the second coupling; and a controllerconfigured to control a scanning position of the reading unit byapplying a drive control signal to the drive source, wherein thecontroller starts controlling the scanning position of the reading unitbased on the transmission ratio changed via the second coupling afterthe detector detects the connected state of the second coupling afterthe selector changes selection from the first coupling to the secondcoupling.
 2. The image reading apparatus according to claim 1, whereinthe selector is configured to exert a pushing force in a direction tobring one of the first and second couplings into the connected state. 3.The image reading apparatus according to claim 1, wherein the firstcoupling includes a first member and a second member, wherein the secondcoupling includes a third member and a fourth member, wherein thetransmission unit includes a first transmission portion configured totransmit the driving force from the first member to the scanning driveunit, and a second transmission portion configured to transmit thedriving force from the fourth member to the scanning drive unit, atransmission ratio of the second transmission portion being differentfrom that of the first transmission portion, and wherein the selectorselectively exerts a pushing force to an output shaft in an axialdirection thereof, the output shaft being connected to the second memberand the third member and configured to output the driving forcetransmitted from the driving source.
 4. The image reading apparatusaccording to claim 1, wherein, when the selector changes selection fromthe first coupling to the second coupling, the connected state of thefirst coupling is removed, and wherein the controller controls thescanning position of the reading unit by inhibiting the scanning driveunit from performing scanning drive according to the drive controlsignal applied to the drive source until the detector detects theconnected state of the second coupling after the selector changesselection from the first coupling to the second coupling.
 5. The imagereading apparatus according to claim 3, wherein the connected state ofthe first coupling remains until the detector detects the connectedstate of the second coupling after the selector changes selection fromthe first coupling to the second coupling, and wherein the controllercontrols the scanning position of the reading unit by driving thescanning drive unit via the first transmission portion according to thedrive control signal applied to the drive source until the detectordetects the connected state of the second coupling after the selectorchanges selection from the first coupling to the second coupling.
 6. Theimage reading apparatus according to claim 3, further comprising a homeposition sensor configured to detect a home position of the readingunit; wherein the transmission ratio of the first transmission portionis larger than that of the second transmission portion, and wherein thecontroller performs position adjustment at the home position in a statein which the connected state of the first coupling remains.
 7. The imagereading apparatus according to claim 6, wherein the controller moves thereading unit from the home position to a change position and stops thedriving source in a state in which the connected state of the firstcoupling remains, and wherein after the selector changes selection fromthe first coupling to the second coupling at the change position, thecontroller starts driving the drive source and causes the reading unitto reads an image of the document.
 8. The image reading unit accordingto claim 6, wherein after the reading unit has completed reading animage of the document, the controller causes the selector to changeselection from the second coupling to the first coupling andsubsequently controls the reading unit to return to the home position.9. The image reading apparatus according to claim 1, wherein the drivesource includes a stepping motor, and wherein the drive control signalincludes a drive pulse to be applied to the stepping motor to controlthe scanning position of the reading unit.
 10. The image readingapparatus according to claim 1, wherein the drive source includes adirect current (DC) motor, wherein the DC motor includes a rotaryencoder, and wherein the drive control signal includes an output pulsefrom the rotary encoder to control the scanning position of the readingunit.
 11. The image reading apparatus according to claim 3, wherein theselector includes: a worm gear driven by a change motor; a cam connectedto a gear driven by the worm gear; a change arm engaged with the cam andconfigured to move the output shaft in the axial direction thereof; anda spring configured to push the output shaft against the change arm. 12.A method for controlling an image reading apparatus configured to readan image of a document by causing a scanning drive unit to performscanning drive of a reading unit, the image reading apparatus includinga speed change mechanism provided in a transmission unit that transmitsa driving force from a drive source to the scanning drive unit, thespeed change mechanism including first and second couplings configuredto selectively come into a connected state to transmit the driving forcefrom the drive source to the drive unit, a selector configured to selectone of the first and second couplings, the method comprising: stoppingdriving of the drive source and causing the selector to change selectionfrom the first coupling to the second coupling; after the selectorchanges selection from the first coupling to the second coupling,detecting the connected state of the second coupling; and after theconnected state of the second coupling is detected, starting controllinga scanning position of the reading unit by applying a drive controlsignal to the drive source based on transmission of the driving force bythe second coupling.
 13. The method according to claim 12, wherein thedetection of the connected state of the second coupling is accomplishedusing a detector.
 14. The method according to claim 12, wherein thefirst coupling includes a first member and a second member, wherein thesecond coupling includes a third member and a fourth member, wherein thetransmission unit includes a first transmission portion configured totransmit the driving force from the first member to the scanning driveunit, and a second transmission portion configured to transmit thedriving force from the fourth member to the scanning drive unit, atransmission ratio of the second transmission portion being differentfrom that of the first transmission portion.
 15. The method according toclaim 14, further comprising: causing the selector to exert a pushingforce to an output shaft in an axial direction thereof, the output shaftbeing connected to the second member and the third member and configuredto output the driving force transmitted from the driving source.
 16. Themethod according to claim 13, further comprising: controlling thescanning position of the reading unit by inhibiting the scanning driveunit from performing scanning drive according to the drive controlsignal applied to the drive source until the detector detects theconnected state of the second coupling after the selector changesselection from the first coupling to the second coupling.
 17. The methodaccording to claim 13, further comprising: controlling the scanningposition of the reading unit by driving the scanning drive unit via thefirst transmission portion according to the drive control signal appliedto the drive source until the detector detects the connected state ofthe second coupling after the selector changes selection from the firstcoupling to the second coupling.